This invention relates to high solids urethane clearcoat compositions, the flow characteristics of which are modified by the incorporation of a flow control system composed of urea thixotrope/acrylic microgel additives.
Earlier work by two of the inventors hereof led to the development of novel, ultraviolet (U.V.) stable compositions which are the subject matter of U.S. Pat. No. 4,387,194, assigned to General Motors Corporation, issued June 7, 1983. One such composition that was used in the current invention is a blend of polypropylene oxide polyethers and a hydroxy-terminated polyurethane derived from the polyethers, a U.V. stabilizing group that is incorporated into the polyurethane polymer backbone, and a melamine-formaldehyde crosslinking agent. These resins are generally the bases of high solids coating compositions forming durable, high gloss finishes useful in painting automobile bodies.
For high solids coatings, the control of their flow behavior, i.e., the competing processes of sagging and leveling, is probably the most important and the most difficult aspect of their formulation. The problem is particularly acute when the paint is to be sprayed onto a vertically oriented surface. Coatings must be able to level in order to minimize surface irregularities or waviness and to develop the necessary appearance properties required of an automotive finish. During this time, various degrees of sagging will occur. Since those factors that govern sagging and leveling necessarily oppose each other, designing a system with complete leveling will result in severe sagging, whereas complete sag control results in no leveling. Consequently, a compromise must be made by balancing those factors that affect sagging and leveling. Our invention provides a method of controlling the flow behavior of a high solids urethane clearcoat by the addition of a combination of a urethane-urea thixotrope/acrylic microgel flow control system.
When working with conventional low solids coatings, a paint-formulator relies almost exclusively on the careful blending of organic solvents to control the rate of solvent evaporation which, in turn, controls the coating's viscosity and rheological behavior from application through the final film formation or cure. These coatings can lose as much as 50 to 70% of their solvent content during the application and flash periods. High solids coatings, however, lose very little solvent during application and flash. Consequently, viscosity increase via solvent loss is negligible and is not a viable means of obtaining flow control.
High solids clearcoats are generally easy to apply to substrates by the spraying technique even though the nonvolatile solids (resin) content of such systems may be well above 50%. However, their viscosities and thixotropic properties are such that after application, they are prone to run and sag when applied to vertically oriented substrates. Improved flow control of these paint compositions is therefore critical to their greater utility in automotive applications where coatings may be sprayed onto vertically oriented body panels or parts with complex surface contours.
Desired rheological behavior for a sprayed paint coating dictates that it has a very low viscosity when subjected to high shear in the paint spraying process. However, once the paint has been applied to a substrate, the viscosity should increase enough so that the paint does not sag or drip but not so much that it fails to level.
Some flow control agents such as N,N-diethanolurea-terminated polyurethane have been used successfully to control sagging in high solids pigmented urethane paint. This practice is disclosed in a copending application U.S. Ser. No. 633,025, filed July 20, 1984 in the name of William T. Short et al and assigned to the assignee of this invention. For instance, in urethane paint containing titanium dioxide pigment, the addition of a single thixotropic flow control agent dramatically improves the flow characteristics by reducing sag to as little as 12% of the control coating which contained no thixotrope additives. However, when the same thixotropic flow control agent is used in high solid polyurethane clearcoats containing no pigment, the effect of the thixotrope on sagging is only marginal.
Accordingly, it is an object of this invention to provide a flow control system (i.e., where two chemical constituents act in concert) which, when added to high solids polyurethane clearcoat, improves the flow characteristics of such clearcoats, particularly to promote ease of spraying and adequate leveling while inhibiting sagging and dripping.
It is another object of this invention to provide a high solids, non-pigmented urethane clearcoat formulation containing a flow control system such that the clearcoat is sprayable in a production environment, curable in a relatively short period of time at relatively low temperatures, and spreadable when wet without sagging or dripping.